1/
#include <fstream>

std::ifstream::pos_type filesize(const char* filename)
{
    std::ifstream in(filename, std::ifstream::ate | std::ifstream::binary);
    return in.tellg(); 
}


2/

#include <fstream>

int getFileSize(const std::string &fileName)
{
    ifstream file(fileName.c_str(), ifstream::in | ifstream::binary);

    if(!file.is_open())
    {
        return -1;
    }

    file.seekg(0, ios::end);
    int fileSize = file.tellg();
    file.close();

    return fileSize;
}